The invention concerns a method and device for surface treatment of substrates with the aid of a gas discharge.
In surface treatment of flat substrates by means of a gas discharge, such as low-pressure glow discharges, methods are known in which the discharge is maintained by means of a microwave antenna, a high-frequency electrode, or a pulsed or timewise continuous voltage, applied to the substrate. Substrate surfaces and counter-electrodes and microwave antennas are thereby mostly arranged opposite each other.
A critical disadvantage of this method is that as a rule, only a low plasma density can be generated and the rate of plasma cleaning or plasma coating of the substrate surface is therefore low. Although the plasma density can be also be increased by increasing the pressure, the associated decrease in the mean free path leads to the transport of materials to and from the substrate surface being strongly hindered. In addition, the tendency of the discharge to local contraction and instability grows. Also disadvantageous in this method is the fact that an undesirable coating of microwave-coupling windows or high-frequency electrodes arises, whereby the coupled power clearly decreases over time.
Also disadvantageous is the fact that large amounts of starting materials are thereby lost and that other internal surfaces of the vacuum chamber become coated in addition to the substrate.
Surface treatment of running metal bands, such as sheet steel or aluminum, activated or supported by an electric gas discharge, presents special problems in batch processes involving the treatment of substrates.
On the one hand, the high running speed of the band requires very high stationary coating rates and plasma densities, for sheet steel up to a rate of 100 m/min. For example, in order to deposit a coating thickness of 100 nm at a band speed of 100 m/min and a coating-zone length of 1 m, a stationary coating rate of 10 μm/min is required. This is about 2 orders of magnitude more than can be achieved with ordinary DC or AC glow discharges.
Plasma densities as high as possible are also to be strived for in order to achieve higher deposition rates for effective removal rates for surface contaminants (oils, fats, waxes) with formation of gaseous products on a rapidly running band. Ordinary glow discharges generally do not have a sufficient degree of ionization and have too low a proportion of active species such as oxygen atoms or hydroxyl radicals.
In addition to providing high plasma densities, production systems of this kind are expected to be able to be operated for several days without maintenance. A condition for this is that parasitic deposition of layers, i.e. the growth of layers in other places than on the sheet metal to be treated, be kept low. It should be considered that in 100 h the hypothetical “stationary” layer thickness on sheet metal at rest is up to 6 cm at a growth rate of 10 μm/min. Even if the parasitic growth rate on a counter-electrode, a deflector, or housing wall is only 1% of this value, the resulting layers with layer thickness of 600 μm would be unacceptable, since because of their internal tensions they would no longer adhere to their substrates and would disturb the coating process in the form of dislodged chips.